At many locations, maintaining security of access requires screening subjects who enter the location for contraband substances. For example, at airports, passengers and luggage are screened for narcotics and explosives. Similar screening is performed at other locations where security of access is important, such as train stations, border crossings, public buildings, government offices, sporting facilities, tourist attractions, mail depots, etc. The subjects to be screened may be persons, parcels, packages, baggage, electronic devices, tickets, and any other subjects which may have come into contact with a target substance.
At locations where subjects must pass through at a high rate, screening may create a bottleneck. It is therefore desirable at such locations to screen subjects quickly. It is also often desirable to screen subjects in a minimally-intrusive way to avoid any unnecessary invasion of privacy.
Trace chemical screening devices detect a target substance, such as a narcotic or an explosive chemical, based on the presence of minute quantities of molecules or ions from residues of the target substance. Thus, compared to metal detectors which screen only for metallic contraband such as weapons, and x-ray machines which screen based on bulk shapes that resemble contraband, trace chemical screening devices may be used to screen for a broader range of target substances, while performing screening at greater sensitivity and selectivity. Trace chemical screening devices may therefore be used in place of, or in conjunction with conventional detection devices such as metal detectors and x-ray machines.
An exemplary trace chemical screening devices is disclosed in U.S. Pat. No. 7,458,283 to Nacson et al. According to Nacson et al, analyte is sampled from the subject by wiping the surface of the subject with a swab. The device then uses a spectrometer to analyze swabbed analyte to determine if a target substance is present.
Trace chemical screening devices which screen subjects without requiring any physical contact with the subject, and are therefore less intrusive than the device disclosed in Nacson et al., are also known. Exemplary non-contact trace chemical screening devices are disclosed in U.S. Pat. No. 5,915,268 to Linker et al. and U.S. Pat. No. 6,610,977 to Megerle. According to Linker et al. and Megerle, analyte is collected from a subject by blowing air onto the subject to entrain analyte into an airflow. Analyte may include residues of the target substance in particle or vapour form. The airflow containing the analyte is sampled, and a detector is used to analyze analyte within the sampled air to determine if a target substance is present. Linker et al. and Megerle disclose a variety of detectors, including detectors which perform detection using ion mobility spectrometry, electron capture detection, and gas chromatography/chemiluminescence.
One problem associated with conventional non-contact trace chemical screening devices lies in providing a detectable concentration of the target substance to the detector. The concentration of the target substance carried in the sampled air may often fall below the detector's sensitivity threshold. As such, a preconcentrator may be used to increase the concentration of the target substance to level above the detector's sensitivity threshold.
However, when a preconcentrator is used, sampled air containing analyte is not provided directly to the detector. Rather, sampled air is first passed through the preconcentrator, which forms a concentrated sample by accumulating analyte from the sampled air over time. For example, the preconcentrator may include an activated carbon filter to absorb analyte from sampled air passing through the preconcentrator.
After the preconcentrator accumulates a sufficient quantity of analyte, the concentration of the target substance in the concentrated sample is increased to a level above the detector's sensitivity threshold. The concentrated sample is then provided to the detector to detect the target substance.
While a preconcentrator enables non-contact trace chemical screening devices to detect low concentrations of the target substance in sampled air, the use of a preconcentrator introduces a number of problems. Firstly, the use of a preconcentrator may consume materials such as activated carbon, thereby increasing operational costs. Secondly, the use of a preconcentrator increases screening time per subject, as additional time spent accumulating sufficient quantity of analyte, thereby reducing detection throughput.
Accordingly, there remains need for an improved non-contact chemical screening device.